The invention relates to an apparatus for degassing thermoplastic synthetic plastics material which is supplied through at least one intake opening into a housing of a first screw section, at the end of which the plasticized synthetic plastics material is pressed into at least one connection channel in which at least one filter for the synthetic plastics material is disposed, from which the synthetic plastics material streams to an upstream opening of a housing of a second screw section which is disposed coaxially to the first screw section and conveys the synthetic plastics material along a degassing region provided with at least one degassing opening to an exit, a sealing section being interposed between the two screw sections which avoids bypassing of the connection channel, the two screw sections and the sealing section being bearingly supported in bores of the housings having equal diameters.
Such an apparatus has become known from a prior suggestion of the inventors (WO 93/04841). This known apparatus enables one to clean thermoplastic synthetic plastics material in plasticized condition by means of the filter and then to degas it, however it is sensitive with respect to disposal of the opening through which the plasticized synthetic plastics material, after having passed the filter, flows into the second screw section. Further, some kinds of synthetic plastics material are only incompletely degassed, inasmuch the synthetic plastics material is subject to turbulences when passing the filter, so that gas inclusions are worked into the interior of the synthetic plastics material rope. Investigations of the applicants have been shown that the problems when degassing are based on the fact that the migration path for the gas bubbles in the degassing region is too long. It is possible to counteract this by operating the screw in the degassing region with a reduced speed so that the synthetic plastics material passes the degassing region comparatively slow. However, this has the disadvantage that the throughput of the apparatus is unsufficient. Another possibility is to make the degassing region comparatively long, what has the disadvantage of a considerable entire length of the apparatus.
The invention has at its object to avoid these disadvantages and to improve degassing at an apparatus of the initially described kind without that the throughput of the apparatus is substantially reduced and without that the constructional length of the apparatus is increased. The invention solves this task by the features that in the housing of the second screw section at least one baffle member for the synthetic plastics material flow is provided between the upstream opening and the degassing openings, which baffle member forces the synthetic plastics material stream into a hose-formed shape and is adjustable with respect to the upstream opening, the baffle member being disposed so in front of the upstream opening that the synthetic plastics material streaming from the upstream opening into the housing of the second screw section hits directly the periphery of the baffle member which is disposed axially spaced apart from the nearest degassing opening and has a cross section tapering towards the second screw section. This baffle member forms a radial gap between its periphery and the inner surface of the housing of the second screw section, which gap as a rule is so narrow that through this gap so to speak a hose-shaped synthetic plastics material film having a small wall thickness is extruded. This hose-shaped synthetic plastics material stream is taken along by the second screw section and is conveyed further in the degassing section, the small wall thickness of this synthetic plastics material film causes a short migration path for the gas bubbles and thereby an efficient degassing, even if the screw revolves with a comparatively high speed. Within this it is of advantage that already filtered synthetic plastics material is supplied to the region of the baffle member, so that foreign matter cannot adversely affect the gap formed between the baffle member and the screw housing. On the other hand, it is efficiently counteracted to the inclusion of the gas bubbles caused by the filter, and this by the fact that the synthetic plastics material which shows gas inclusions caused by the filter, after being supplied again into the housing of the second screw section, at first streams around the baffle member and substantially only then is conveyed further by the screw of the second screw section. This flow around the baffle member offers to the gas inclusions a longer period of time for loosening from the synthetic plastics material. Therefore, the time interval which is at disposal for the gas inclusions to leave the synthetic plastics material, is increased so that also comparatively long migration paths of the gas inclusions are taken into account.
Within constructions of another kind, baffle members are known which give the synthetic plastics material rope supplied by a screw a hose-shaped form in order to increase degassing (DE-A 2,237,190, DE-A 2,031,935, EP-A 119,029). Within the known constructions, however, no filtering of the synthetic plastics material is provided so that the object on which the invention is based, does not exist there.
Basically, some baffle members may be provided within the spirit of the invention, which are disposed one behind the other, when seen in conveyance direction of the synthetic plastics material. If desired, also a baffle member composed from a plurality of sections may be used. Preferably however, within the spirit of the invention only one single baffle member is provided, around which the synthetic plastics material which enters the housing of the second screw section, flows in the region of the inlet opening, so that the synthetic plastics material is transferred into its hose-like shape.
According to a preferred embodiment of the invention, the screw threads of the second screw section reach up into the region between the baffle member and the nearest degassing opening, preferably the screw threads immediately neighbour the baffle member. Thereby it is avoided that the synthetic plastics material hose collapses again or, respectively, that substantial thickenings of the synthetic plastics material after its formation into its hose-like shape, may occur, what would hinder the degassing effect.
By adjustment of the baffle member in axial direction of the two screw sections, the effective cross section of the inlet opening can be changed, if the tapered section of the baffle member comes into the region of this inlet opening. The circumstances of forming the hose-shaped synthetic plastics material film, therefore, can be influenced at choice, in particular the wall thickness of this film. Within this, it is particularly favourable within the spirit of the invention, to give the tapered cross section of the baffle member the shape of a conical section of the baffle member, that is a frustoconical surface. This on the one hand favours loosening of the synthetic plastics material film from the baffle member, on the other hand there is the possibility, as already mentioned, to steadily continuously change the radial width of the annular gap formed between the periphery of the baffle member and the inner surface of the housing of the second screw section by axial displacement of the baffle member. Within this, it is particularly favourable within the spirit of the invention, if the baffle member--when seen in flow direction of the synthetic plastics material--is connected to the core of the screw threads of the second screw section. If so, the baffle member rotates with the neighbouring screw, what favours the distribution of the synthetic plastics material supplied through the inlet opening to the shape of a hose. Furthermore, the advantage is obtained that the axial displacement of the baffle member can be made by means of the neighbouring screw, and for this according to the invention the construction is so chosen that the second screw section connected to the baffle member is shiftable in its axial direction, preferably by means of a screw engaging the front end of this screw section. Alternatively, however, within the spirit of the invention the construction can also be chosen so that the baffle member is axially displaceably disposed on the second screw section and is connected to a separate adjustment means. Also within such a construction the feature can be maintained that the baffle member rotates together with the neighbouring screw section.
It is of particular advantage within the spirit of the invention, if the baffle member is detachably connected to the second screw section and can be exchanged with baffle members of different dimensions. Thereby, the conditions at the inlet opening can be changed at choice and can be adjusted to the given requirements of the material.
Investigations of the inventors have further shown that problems may arise if the synthetic plastics material supplied to the first screw section is only incompletely comminuted, so that bigger pieces or shreds remain in this material which are only incompletely plasticized by the first screw section before they are pressed into the connection channel. Such portions of the synthetic plastics material which are not completely plasticized may block the connection channel or the filter inserted into it. The same can be caused by foreign bodies contained in the synthetic plastics material, for example metal pieces or stones, textile portions and the like. Such an obstruction in the connection channel or a blocking of the filter causes a decrease of the synthetic plastics material flow in the section of the connection channel following the filter, and thereby also in the inlet opening leading the synthetic plastics material to the baffle member, whereby forming of the synthetic plastics material hose could be impared or destroyed. In order to overcome these difficulties, according to a further embodiment of the invention the construction is so chosen that in the region of the first screw section neighbouring the inlet opening of the connection channel the screw is provided with teeth comminuting the material conveyed by it. These teeth which rotate together with the first screw section, comminute the synthetic plastics material disposed in front of the inlet end of the connection channel and also the impurities contained in it, and this just at the critical location. By the teeth the unplasticized synthetic plastics material portions as well as bigger foreign body portions of the material entering the connection channel are so comminuted that this channel cannot more be plugged. If the particles obtained by the comminution are not so small that they can pass the filter, they are collected by the filter and are removed from the apparatus when the filter is exchanged.
According to a preferred variant of the embodiment, the teeth are formed in one single piece with the core of the screw section. Therefore, they are worked from the material of this core. For manufactoring reasons, however, it is easier according to the preferred embodiment of the invention, if the teeth are provided on a separate carrier member which is connected for common rotation with the core of the screw section. This carrier member, for example, can be screwed to the core of the screw section.
Preferably within the spirit of the invention the teeth are formed as ribs which extend up to the inner surface of the housing of the screw section. These ribs, so to say, mill the material to be comminuted. They may extend parallel to the axial direction of the screw section, however, if desired, also along helix lines or the like, whereby an oblique cut onto the material to be comminuted is obtained. Particular favourable results are obtained within the spirit of the invention, if the teeth overlap the inlet opening entirely, when seen in axial direction of the screw section, preferably overdimensioned before and behind. Then, when seen from above, the inlet opening is completely covered by the teeth, and the teeth cooperate with the edge of the inlet opening, which edge acts as a shearing edge. In order to optimally meet the stresses occuring there, within the spirit of the invention, a wearing part is inserted into the wall of the inlet opening.
Further features and advantages of the invention can be seen from the description of examplative embodiments, schematically shown in the drawings.